In copending application Ser. No. 917,920, filed June 22, 1978, entitled "Harmonic Formant Filter for an Electronic Musical Instrument", by the same inventor as the present application and assigned to the same assignee, and hereby incorporated by reference in the present application, there is described a system for implementing a fixed formant filter in a polyphonic tone synthesizer of the type described in U.S. Pat. No. 4,085,644. As described in the above-identified application, the attenuation versus frequency characteristic of the fixed formant filter is controlled by a set of scale factors stored in an addressable "read only" memory (ROM). While the number of words stored in the ROM is limited to 96 to implement one formant filter, it is desirable to store a number of such sets of words to permit the player of the instrument to select from a wide variety of formant filter characteristics. Assuming that each word in the memory is 6 bits in length and that it is desired to store data for up to 10 different formant filter characteristics, a ROM memorry having a capacity of 5760 bits would be required.
The present invention is directed to an arrangement for providing a wide selection of formant filter characteristics by synthesizing a set of scale factors as needed to produce a selected fixed formant filter. This permits the number of stored data bits to be greatly reduced and at the same time provides greater flexibility in choosing the formant filter response characteristic.
The concept of approximating formant filters having a low-pass, high-pass or Q-accent characteristic, is described for example, in U.S. Pat. Nos. 4,000,675 and 4,026,179. These patents disclose formant filters for use in tone synthesizers in which a series of straight line segments of attenuation v. frequency are used to approximate the desired response. The straight line segments are stored in the form of data consisting of the coordinants of the breaking points of the linear segments as well as the slopes of these segments. However, such known systems for synthesizing a formant filter requires large amounts of data to be stored for more complex filter characteristics.